The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Low pressure radio frequency (RF) generated plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density, in the range of 1011-1013 ions/cm3.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
Although such techniques can reduce the formation of voids, further reduction of void formation is needed.
It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter depositing a layer which enhances both sidewall and bottom coverage.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention, a plasma generating apparatus in which a layer of titanium, a titanium compound or other suitable deposition material is deposited in such a manner as to increase the coverage of sidewalls of channels, vias and other high aspect ratio openings and structures having a sidewall in a substrate. It has been found that by increasing the sidewall coverage of underlayers, the flow of aluminum or other overlayer materials into the opening is enhanced so as to substantially reduce the formation of voids in the overlayer.
In one embodiment, increased sidewall coverage by an underlayer material is achieved by generating an ionizing plasma in a relatively low pressure precursor or sputtering gas. By reducing the pressure of the sputtering gas, it is believed that the ionization rate (or the directionality or both) of the underlayer deposition material passing through the plasma is correspondingly reduced which in turn is believed to increase the sidewall coverage by the underlayer. Although the ionization rate is decreased, sufficient bottom coverage of the channels by the underlayer material is maintained. Another advantage of reducing the sputtering gas pressure is that the deposition rate of the underlayer material may be increased as well.
In an alternative embodiment, increased sidewall coverage by the underlayer material may be achieved even in a high density plasma chamber by generating the high density plasma only during an initial portion of the underlayer material deposition. It has been found that good bottom coverage may be achieved by ionizing the underlayer deposition material using a high density plasma during the initial portion of the deposition. Once good bottom coverage has been achieved, the RF power to the coil generating the high density plasma may be turned off entirely and the remainder of the underlayer deposition conducted without the high density plasma. It has been found that good sidewall coverage is then achieved in the latter part of the deposition. Consequently, good overall coverage of the opening is achieved combining the bottom coverage of the initial portion of the deposition with the sidewall coverage obtained during the latter portion of the underlayer deposition.